Manuel D. Mills' U.S. Pat. No. 3,851,995 sets forth a pump-off control apparatus for a pumpjack unit. The control device senses a change in the normal movement of the rod string on the downstroke of the pumping cycle. As the fluid level within the borehole falls below the inlet of the downhole pump means, the occurrence of fluid pounding manifests a shock-like reaction in the rod string which is transferred uphole to the walking beam of a pumpjack. The sensing apparatus measures this change in movement and converts the fluid pounding reaction into a signal for a control circuit. The control circuitry is connected to de-energize the prime mover of the pumpjack apparatus for an interval of time which allows the fluid level in the borehole to recover.
The U.S. patents to Montgomery, et al. U.S. Pat. Nos. 3,817,094 and 3,838,597 illustrate well monitoring apparatus by which a load transducer is secured to the surface structure of the walking beam of a pumpjack unit, so that the transducer senses a change as the walking beam is bent or distorted due to the rod string load. The transducer generates a signal representative of load changes in the structure as the walking beam is rocked.
Reference is made to the above issued patents, to my previous patents, to the references cited during the prosecution thereof, and to the field of search referred to therein.
______________________________________ REFERENCE TO THE PRIOR ART ______________________________________ 2,070,320 3,851,995 2,107,151 3,926,047 2,163,665 3,951,209 3,817,094 3,965,736 3,824,851 4,043,191 3,838,597 4,058,757 4,142,546 ______________________________________
The present invention differs from the known prior art by the provision of means by which a signal is generated in response to movement between a main structural component of a pumpjack unit and a relative immovable other structure. The signal amplitude varies concurrently with the variation in stress and strain to which a pumpjack structural component is subjected. The structural component is deflected in a cyclic manner analogous to the cyclic and oscillatory movement of the walking beam of a pumpjack unit. The deflection is synchronized with, but not necessarily in phase with, the up and down stroke of the rod string. Nevertheless, the cyclical occurring deflection occurs one cycle for each reciprocal movement of the rod string, and therefore the resulting signal can be related to both position and tension associated with the rod string, for the reason that the magnitude of the deflection, or more exactly the change in magnitude of the deflection, is a direct result of the varying geometry of the pumpjack unit required to produce the oil well. This concept is not found in the foregoing patents cited herein.